The present invention relates to a magneto-optical recording material for use in a recording layer of a magneto-optical recording medium.
Recently considerable attention is placed on a magneto-optical recording medium comprising a magnetic thin layer in which information is recorded by forming magnetic domains therein by utilizing the thermal effects caused by the light beams projected thereto and from which the recorded information is read out by utilizing the magneto-optical effect of the magnetic thin layer.
As a material for use in a conventional magneto-optical recording medium, various amorphous materials comprising rare-earth metals and transition metals are known. A magneto-optical recording medium in which a magnetic material made of such an amorphous alloy is employed is usually fabricated, for instance, by depositing a Tb-Fe alloy on a substrate such as a glass plate by vacuum evaporation or by sputtering to form a magnetic layer with a thickness of about 0.1.about.1 .mu.m.
Such amorphous alloy magnetic materials, in particular, the rare-earth metal components contained therein, however, are so susceptible to oxidation and corrosion that they have the shortcomings that the magneto-optical characteristics of a magnetic film layer made of such an amorphous alloy magnetic material considerably deteriorate with time and the oxidation and corrosion of the magnetic film layer are accelerated by the light applied thereto and the heat generated therein at the time of recording information. Amorphous alloy magnetic materials have the further shortcoming that they are apt to be crystallized when heated, so that the magnetic characteristics easily deteriorate.
Further, a magnetic film made of an amorphous alloy magnetic material has a low transmittance ratio in the wavelength region of the laser beams for recording information, so that the recorded information is read out by utilizing the magneto-optical effect caused by the laser beams being reflected by the surface of the magnetic film layer, that is, the Kerr effect. However, the Kerr rotation angle thereof is generally so small that it has the shortcoming that the reproduction performance is low.
The inventors of the present invention previously proposed in Japanese Laid-Open Patent Application No. 59-45644 a magneto-optical recording medium including a magnetic layer comprising a magnetic material represented by a general formula (A) or a magnetic material represented by a general formula (B) in view of the fact that a magneto-plumbite type barium ferrite is excellent in the stability with time and in the transparency in the laser beam wavelength region, so that it is expected that the Faraday effect thereof can be utilized: EQU MeM.sub.x M.sub.y Fe.sub.12-p O.sub.19 (A)
wherein Me represents at least one element selected from the group consisting of Ba, Pb, Sr and Sc, and M represents at least one element selected from the group consisting of Co, Mn, Ti, Zn, Al, Sn, Cu, Cr and Mg, p=x+y (y can be zero (0)), and 1.2.ltoreq.p.ltoreq.2. EQU CoM.sub.z Fe.sub.2-z O.sub.4 (B)
wherein M represents at least one element selected from the group consisting of Co, Mn, Ti, Zn, Al, Sn, Cu, Cr and Mg, and 0.75.ltoreq.z.ltoreq.1.3.
BaFe.sub.12 O.sub.19 has high magnetic anisotropy. However, its magneto-optical effect is so small that it cannot be used as magneto-optical recording material in practice.
The magneto-optical effect of BaFe.sub.12 O.sub.19 can be significantly improved by replacing part of the Fe atoms in BaFe.sub.12 O.sub.19 with Co atoms. However, in this case, charge compensation becomes necessary because 3-valence Fe is replaced by 2-valence Co. If charge compensation is performed by replacing part of Fe atoms with a 4-valence metal, the crystalline anisotropy decreases.
FIG. 4 and FIG. 5 show the above facts. In these figures, M represents a 4-valence metal, x and y each represent a substitution number, and H.sub.A represents the anisotropy field of the crystalline anisotropy.
Thus, when Co is added, the magnetic anisotropy decreases in the end. Therefore, there is a certain limitation to the amount of Co that can be used for such replacement. Thus, the magneto-optical effects cannot be sufficiently improved by more addition of Co. When the magnetic anisotropy is small within the range in which a film magnetized perpendicular to the plane thereof can be formed, the square ratio (residual magnetization (Mr)/saturation magnetization (Ms)) of the hysteresis loop of the magnetic film which is not subjected to demagnetizing field correction becomes improper. The result is that suitable characteristics for magneto-optical memory device cannot be obtained.
When recording of information is performed in a magneto-optical recording material by a light modulation system, it is required that the square ratio (hereinafter referred to as the Mr/Ms ratio) in the hysteresis loop be 1 in the case where no demagnetizing field correction is performed. When the Mr/Ms ratio is small, even if the Faraday rotation (.theta..sub.F), which is a predominant magneto-optical effect in this case, is large, the available readout performance will be at most the Mr/Ms ratio.times..theta..sub.F.
FIG. 6A shows an ideal hysteresis loop.
FIG. 6B shows the hysteresis loop of BaFe O.sub.19.
FIG. 6C shows the hysteresis loop of a representative BaCo.sub.x M.sub.y Fe.sub.12-(x+y) O.sub.19.